The use of chemotherapeutic drugs as an adjuvant to external beam radiotherapy, surgery, or other treatment modalities is common practice for the treatment of a wide variety of solid tumors. This approach has demonstrated some success in the management of certain cancers. The rationale for combining chemotherapeutic agents with external beam radiotherapy is to radiosensitize the irradiated tumor tissue and/or to target subpopulations of malignant cells that have metastasized from the primary lesion demarcated for beam therapy. Although the tradition of chemoradiotherapy has been practiced for decades and shows promise, some attempts have not succeeded in demonstrating either an added therapeutic benefit or a reduction of normal tissue toxicity. In another approach, radiolabeled chemotherapy agents have been used in an attempt to achieve enhanced cytotoxicity both in human cancer cells and apparently normal hamster fibroblasts. Chemotherapy has also been combined with radioimmunotherapy.
One limitation of chemoradiotherapy is the frequent lack of interaction between chemotherapeutics and ionizing radiation. This often leads to escalation of radiation and drug doses, which in turn, results in elevated normal tissue toxicity. Moreover, lack of specificity of chemotherapy drugs for tumor tissue can result in an insignificant difference in toxicity towards malignant and normal tissues thereby providing no added therapeutic benefit compared to surgery and radiation alone. Despite these limitations, chemoradiotherapy often provides considerable therapeutic benefit. However, observed inconsistencies in treatment outcomes may be due to the widely varying chemotherapeutic drug concentrations employed and radiation absorbed doses achieved. In addition, there is evidence demonstrating that optimization of radiation dose and drug concentration, and the time sequence for administering drugs and radiation play important roles in treatment responses both in vitro and in vivo. Also, regardless of the quality of radiation used, the wide variability in drug toxicity in normal cells of different histologies has to be considered in favor of the most sensitive tissue in chemoradiotherapy. Unfavorable outcomes in therapies involving the use of chemotherapy drugs and radiopharmaceuticals have been attributed to insufficient tumor specificity, poor tumor vascularization, and nonuniformities in agent distribution at the macroscopic, cellular, and subcellular levels. Determination of drug and radionuclide incorporation at the single-cell level has been difficult. As such, estimation of intracellular chemotherapy drug concentration and intra-cellular radioactivity (required to determine radiation absorbed dose to the cell) has largely been restricted to the macroscopic level. Accordingly, it has been difficult to establish a relationship between therapeutic agent incorporation and biologic response.
It has now been discovered that, even in situations where there is optimum perfusion and no diffusion barriers, cellular incorporation of radionuclides and chemotherapeutic drugs is not only nonuniform, but is also log normal. This strongly suggests that the limited success in chemoradiotherapy of primary solid tumors and metastatic disease is likely due to this log normal phenomenon, in which minute subpopulations of cells take up very little or no therapeutic agent. Repopulation by these subpopulations could mask a possible treatment benefit and result in an even more resistant neoplastic form. Thus, to enhance tumor response, there continues to be a need to address the nonuniform, log normal distribution of chemotherapy drugs and radiopharmaceuticals. Using a quantitative immunofluorescence-based approach, it has now been demonstrated in a 3-dimensional culture system that concomitant measurement of radiopharmaceutical uptake and biologic response in individual cells within a population can be used to predict the response of subpopulations of cells, and ultimately of the entire population. Such capabilities now allow the design of more effective cocktails for clinical applications.